If a developer adds a new field (column) to a database table, what value will this field have for all historical records?

If a developer adds a new field (column) to an existing database table, this field will take the default value for all existing (historical) records, which depends on the data type and database settings:

• For numeric types, it is usually 0.
• For string types, it is an empty string or NULL if the column allows NULL.
• If the column is declared as NOT NULL without a default value, adding it may cause an error.

For example, in PostgreSQL, when adding a column without a DEFAULT value and allowing NULL, the old records will have NULL in that field.